One of the most critical bottlenecks for increasing solar energy utilization is overcoming the inherent intermittency of solar radiation. Converting solar energy into high density chemical energy such as hydrogen is an effective way to address this challenge. In particular, dissociating water with sunlight-driven photoelectrochemical cells (PECs) has received enormous attention. The key challenge for PECs is identifying a combination of materials that absorb the visible solar spectrum, minimize overpotential losses, achieve proper energetic alignment between the light absorber and the electrochemical redox levels, and exhibit excellent stability.
Thus far no one materials system has proven ideal for these stringent requirements, though recent progress is being made on systems such as Si, Fe2O3, Cu2O, etc. One solution is a heterojunction PEC which couples an efficient solar absorber in the 1-1.6 eV band-gap range with an efficient and robust catalytic layer, typically an oxide with a much larger band-gap. In the prior art, heterojunctions including Si—TiO2 Si—Fe2O3, WO3—Fe2O3, WO3—BiVO4 and Cu2O—TiO2 have been explored; however, they have had limited success at room temperature.
While heterojunctions improve harvesting of the solar spectrum, even a small uphill barrier at the interface can severely impede electron transfer and substantially increase the rate of carrier recombination. Moreover, the considerable overpotential required to drive the reaction at a rate matching the solar flux leads to significant losses. Even a state-of-the-art oxygen-evolution catalyst such as RuO2 requires a 200 mV overpotential at a 10 mA cm−2 current density.
An alternate approach to fully harvest the solar spectrum is to forgo semiconductor photo-excitation, and instead convert concentrated solar energy into heat. In thermochemical decomposition of steam, thermal energy rather than electrical energy assists chemical reactions involving energetic barriers. Remarkable reaction kinetics has been reported as a result of the high operating temperature, typically from 1,300 to 1,600° C. However, operating at this temperature range requires specialized solar reactors lined with refractory material and precision solar concentrators (typically at concentration exceeding 1,000 suns, where 1 suns is 1 kW m−2), dramatically increasing the cost. Other approaches aim to separate the thermal and electrical reactions to multiple devices. For example, Licht, et al., proposed to utilize electricity from multi-junction solar cells and heat from unabsorbed concentrated sunlight to drive a high-temperature electrolyzer in Int. J. Hydrogen Energy, Vol. 30, pp. 459-470 (2005), Advanced Materials, Vol. 23, pp. 5592-5612 (2011), Int. J. Hydrogen Energy, Vol. 35, pp. 10867-10882 (2010), and J. Phys. Chem., Vol. 113, pp. 16283-16292 (2009). Another approach combining a photochemical reactor with thermochemical cycles was also proposed by T-Raissi et al., in J. Sol. Energy Eng., Vol. 129, pp. 184-189 (2007). However, the system complexity of the multi-device, multi-step fuel production cycles may ultimately limit their scalability.